Here is how to convert a decimal number with a decimal point to binary(which was taught to us when the Earth had aged a bit). As an example we'll convert .875 as we go along.

1)Take your number and multiply it by 2. So when we do this here we get 1.75 .
2)If the result of the previous step is greater than 1, put down a 1 wherever you are writing the binary expansion, and subtract 1 from the result. So here, we put down a 1 and subtract 1 to get .75
3)Now repeat this process. In general the binary expansion may not terminate so you can take as many places as you like. In this case we multiply by 2 to get 1.5 . So we put down another 1 and take .5 as the new number. Multiplying again we get another 1 but the new number is 0.
So
(.875)10 = (.111)2 .
There, thats it!